In direct current motors, a commutation system is employed which changes the current flow in the stator windings at specific points in the rotor rotation in order to create a rotating magnetic field in the area surrounding the rotor to create the motor's torque. This commutation, or electrical switching, must being synchronization with and in leading angular relationship with the permanent magnets of the motor.
In a typical brushless DC motor, the permanent magnets are placed on the rotor and coils (windings) are int eh stator, Commutation switching to the coils is preformatted by amplified current signals originating at magnetically energized electronic switches, known as Hall effect transistors. When these transistors are exposed to magnetic flux, they alternately toggle "on" or "off " depending on the direction of the magnetic flux passing through its.
Prior art switching transistors 11 as shown in FIG. 2 are positioned around and mounted stationary about the outer periphery of an auxiliary magnet 13 which is fixed to the end of the motor shaft 15. Since the rotor magnets and the auxiliary magnet share a common shaft, the necessary synchronization and angular relationship is maintained. However, as shown in FIG. 6a, the flux pattern of the auxiliary magnet in the prior art device at its theoretical switch point is changing tangent to the desired transistor plane and is therefore indefinite and lies within the "dead band" response of the transistor 11. In addition, any mismatch of field strength of the auxiliary magnet 13 would tilt this flux pattern. The resulting error in switching point effects the angular relationship of the rotating magnetic field with respect to the rotor magnets. This results in an ultimate loss of smoothness of the interaction of the two fields, resulting in both "cogging" and loss of torque. Furthermore, the use of an auxiliary magnet at the end of the rotor shaft requires an additional magnetic structure and increases the overall length of the motor.
Prior art of which the applicant is aware and is pertinent to the present invention are U.S. Pat. No. 4,311,933, entitled "Brush Direct Current Motor", issued to Riggs et al on Jan. 19, 1982; and U.S. Pat. No. 4,494,028, entitled "Integral Coaxial Commutation and Rotor Magnets and Apparatus and Method for Making Same", issued to Brown on Jan. 15, 1985. The disclosures of both of these references are pertinent in that they show inverted-type motors where the stator and switching transistors are within the rotor structure. However, the commutation Hall transistors employed are triggered by auxiliary commutation magnets and there is no groove in the rotor. Furthermore, there is no rotor structure on the opposite side of the Hall transistor to concentrate and direct the magnetic flux disclosed in either reference.